CDMA 20001x is a version of CDMA 2000, which utilizes both circuit networks and packet networks and provides voice service and low-rate data services with a maximum transmission data rate of 307.2 Kbps. CDMA 2000 1xEV-DO is a data-optimized evolution of the CDMA2000 1x standard with High Rate Packet Data (HRPD) technology, which is dedicated for a packet network to transmit only data and provides bi-directional high-rate data services with a maximum transmission rate of 2.4 Mbps. Its evolution of EV-DO RevA has the maximum forward link data rate of 3.07 Mbps and maximum reverse link data rate of 1.8 Mbps. Moreover, EV-DO RevA has the capability to support VoIP service. By time-dividing radio channel into separate pilot, MAC and data channels, EV-DO, which uses a 1.25 MHz data channel, improves spectrum efficiency and eliminates the interference increase that voice traffic would cause data speed to drop.
Hereinafter, the CDMA 2000 1x system is simply referred to as “1x system” or “CDMA system” and the CDMA 2000 1xEV-DO RevA system is simply referred to as “EV-DO system” for the convenience of explanation.
Currently, the EV-DO network has generally been used together with the conventional 1x network. More specifically, the EV-DO network may only cover some cities, or hot spots, whereas the conventional 1x network covers almost everywhere to form a seamless radio data network. Thus, the handoff from VoIP in EV-DO system to 1x circuit voice becomes very important for keeping the voice call continuity.
The existing hand-over from 1xEV-DO to 1x system is related to Circuit Services Notification Application (CSNA). CSNA has been introduced in the EV-DO system to address operation of an Access Terminal/Mobile Station (AT/MS) that otherwise would have to periodically retune to the 1x system to check for pages while it is monitoring the EV-DO system. For CSNA, the AT (a device providing data connectivity to a user, which is equivalent to a mobile station in 1x systems.) and the AN (Access Network, a logical entity in the Radio Access Network (RAN) used for radio communications with AT.) implement a filtering mechanism to allow notifications associated with certain services to be tunneled between 1x and EV-DO systems.
The handoff procedure from 1xEV-DO to 1x system defined in 3GPP2 A.S0008-B as follows:                (1) AN receives a Route Update Message (RUM) for reporting the current link status including the pilot strength, pilot phase of each pilot in active set and neighbor set from an AT/MS, and finds the current pilot strength (Ec/Io) for the EV-DO cell is lower than a predetermined threshold and cannot maintain the VoIP service quality.        (2) AN sends the 1X overhead messages to AT/MS through CSNA protocol.        (3) AN sends the 1X service redirection message to AT/MS through CSNA protocol. This will indicate the AT/MS to begin the procedure to move the call from VoIP of 1xEV-DO to circuit voice of 1X.        (4) AT/MS send the 1X origination to AN through CSNA.        (5) AN sends the handoff direction message to MS/AT.        (6) MS/AT tunes to 1X channel to continue the call.        
Whereas 1x pilot is transmitted continuously, the EV-DO pilot burst has 96 chips duration with only one pilot burst for each half slot. Consequently, the RUM sent by the AT in EV-DO system only includes EV-DO pilots in current system. Thus, the pilot measurement for the 1X system is not performed and the AN cannot know clearly what is the CDMA candidate in fact.
Compared to a current handoff procedure, the VoIP to 1X circuit voice handoff has no pre-provisioning candidates to perform its blind handoff. Therefore, the AN has to determine the active set of 1X system based on the 1X sector information provided by the CSNA protocol during the handoff procedure. The AN will get the 1X neighbor list and sort the list based on the number of occurrence in the overhead messages. Then the 6 pilots of the font candidates of the list becomes the active set of the 1X system.
Since the generated handoff candidates are based on the possibilities of occurrence in the overhead message, and not on the real measurement, the successful handoff cannot be assured. Actually this process cannot tell which CDMA cells have the best coverage for the AT, and it is possible that a CDMA cell has the good coverage but is missed because only 6 pilots are allowed in the active set. Thus the call drop may happen during the handoff.
In addition, another issue for the current system is the “Near-Far Interference” between the border of EV-DO system and 1x system. The EV-DO system is time divided system in the forward link. And the pilot channel transmission is synchronized for the whole system, thus a pilot signal from the neighbor EV-DO cell is the main interference for the EV-DO system. When a cell is a border cell of the EV-DO system, the pilot strength thereof may be near 0 dB since there is no interference of neighbor cell. Therefore the AT in the border EV-DO cell can maintain the link further than in the normal EV-DO cell. Thus there is the possibility that the AT eventually is very near to the CDMA cell when it tries to maintain the long EV-DO traffic link. Then the transmitter of the AT may cause an interference with the reverse link of the CDMA cell, which is called the “Near-Far Interference between EV-DO and CDMA”. It potentially may decrease the CDMA cell's capacity. Of cause, the interference effects are a function of the spectral location of CDMA and EV-DO carriers, for example, the worst interference impact comes from the EV-DO channel spectrally adjacent to a CDMA channel.